Specifications concerning the concentration of aromatic compounds in gas oils will be coming into force in the future.
At the moment, desulphurizing units for this type of feed are not capable of satisfactorily hydrogenating aromatic compounds at the same time.
It is thus highly probable that in the future, new types of units will be developed which will not only carry out intensive desulphurization but will also carry out high efficiency hydrogenation of the aromatic compounds.
These new units may be constituted by two reaction zones. The first reaction zone (which may be a reactor) will reduce the concentration of sulphur to a low value. Then the second reaction zone will hydrogenate the aromatic compounds present in the effluents from the first zone. Catalysts based on noble metals are the best hydrogenation catalysts. Unfortunately, catalysts based on noble metals are usually extremely sensitive to the presence of sulphur.
Our research has led us to explore the possibility of adding chlorine to a feed which may optionally have been desulphurized to increase the hydrogenating activity of the catalyst. Chlorine injection may be continuous or otherwise depending on the desired level of activity.
U.S. Pat. No. 3,639,227 and U.S. Pat. No. 3,816,299 describe processes for the hydrogenation of aromatic compounds in the presence of chlorine.
U.S. Pat. No. 3,816,299 describes the addition of carbon tetrachloride to the hydrogenation zone in a concentration of 0.01-1% by volume (HCl equivalent) with respect to the total gaseous phase. The zone contains a catalyst based on a noble metal and operates between 93C and 538C at 1-300 atm. In the treated feeds, the aromatic compounds are essentially monocyclic and are thus mainly kerosines. Comparative examples III-VI demonstrate that process does not work at all well when chlorinated compounds such as HCl, C.sub.3 H.sub.7 Cl and CH.sub.2 Cl.sub.2 are used instead of CCl.sub.4.
U.S. Pat. No. 3,639,227 also shows that the addition of chlorine to a hydrogenation zone improves the reaction yield in the presence of a catalyst containing a noble metal and about 0.3% of chlorine but containing no fluorine. The quantities of chlorine added, in particular when using C.sub.3 H.sub.6 Cl.sub.2, are in the region of 0.6%. The feeds which can be treated are kerosines or spirits containing at most 0.5% of sulphur.